Switched reluctance motors are characterized in that torque is produced by the tendency of the rotor to align with an excited stator winding, thereby maximizing the inductance of the excited winding. Typically, both the rotor and the stator have unequal numbers of evenly spaced poles. The poles of the stator are wound with field windings, whereas, the poles of the rotor are not wound. Each pair of stator poles positioned opposite each other have field windings that are electrically coupled in series or in parallel so that both field windings are excited simultaneously, thus, comprising a phase of the motor. For producing continuous rotation in a switched reluctance motor, each phase is driven, typically by applying a DC excitation voltage, in an appropriate sequence according to the position of the rotor. This process of driving the phases in sequence is referred to as commutation.
FIGS. 1A-C illustrate a sequence of side sectional views of a switched reluctance motor during counterclockwise rotation of the rotor 10. In FIG. 1A, the rotor poles 1 and 3 are aligned with the stator 12 poles A and A' (phase A). In this position, the windings LA of phase A have a maximum value of inductance. Therefore, to move the rotor 10 into the position illustrated in FIG. 1A, phase A is driven by applying an excitation voltage across the windings LA of phase A, thereby inducing a current in the windings LA of phase A.
If phase A remains excited while the rotor 10 continues to turn past the position of alignment illustrated in FIG. 1A, however, a torque would be produced in a direction opposite the direction of rotation. To avoid this, and because the current cannot instantaneously fall to zero, the excitation voltage is preferably removed from phase A prior to the rotor 10 reaching this position of alignment illustrated in FIG. 1A. The number of degrees of rotation between removing the excitation voltage from a phase and alignment of the rotor 10 with the corresponding phase is referred to as the phase advance.
Upon removing the excitation voltage from phase A, an excitation voltage is applied to the windings LB of the stator 12 poles B and B' (phase B), thus, driving phase B. FIG. 1B illustrates the rotor 10 continuing to turn beyond alignment of the rotor 10 poles 1 and 3 with phase A and towards alignment of the rotor 10 poles 2 and 4 with phase B. Prior to reaching alignment with phase B, as illustrated in FIG. 1C, the excitation voltage is removed from phase B and an excitation voltage is applied to the windings LC of the stator 12 poles C and C' (phase C). Note that as illustrated in FIG. 1C, the inductance of the windings LB of phase B is at a maximum value. Prior to the alignment of the rotor 10 poles 1 and 3 with phase C, the excitation voltage is removed from phase C and an excitation voltage is applied to phase A. This commutation process continues, thereby maintaining continuous rotation of the rotor 10.
Switched reluctance motors, such as the one illustrated in FIGS. 1A-C, differ from brushless DC motors in that switched reluctance motors do not utilize permanent magnets. The use of permanent magnets in the rotor of a typical brushless DC motor tends to increase production costs and can lead to operational problems should the magnets loosen or vibrate. In contrast, the rotor of a switched reluctance motor can be relatively inexpensively manufactured from stamped sheet laminations. In addition, each field winding of a switched reluctance motor is driven independently of the other field windings, whereas, the field windings of a brushless DC motor typically share common terminals, being connected in a delta or wye configuration.
Switched reluctance motors differ from AC induction motors in that AC induction motors generally include field windings in the rotor which tend to increase the cost of producing AC induction motors. As noted above, switched reluctance motors do not have rotor windings.
While switched reluctance motors have relatively low production costs in comparison to other types of motors, they tend to be difficult to control, requiring complicated driving circuitry. This is because the relationships between torque, field current, speed, and optimal phase advance tend to be highly non-linear and also tend to vary with the load.
For driving a switched reluctance motor, knowledge of the rotor position is generally required for appropriately commutating the motor. In prior systems, the rotor position has been determined by position sensors mounted to the rotor shaft. For example, the use of encoders, Hall effect sensors, and sensing transformers are known. These sensors can increase the cost and complexity of such a system and reduce its reliability.
Sensorless control circuits have been developed for switched reluctance motors. For example, U.S. Pat. No. 5,097,190, entitled, "Rotor Position Estimator For A Switched Reluctance Machine," measures phase flux and phase current to derive rotor angle estimates. This system, however, utilizes complex circuitry in that it requires a microprocessor or a pair of analog-to-digital converters and a look-up table to estimate the rotor position.
U.S. Pat. No. 4,520,302, discloses a drive circuit for a stepping motor. Current through a phase winding that is next in sequence to be driven is allowed to increase for fixed period of time determined by a clock pulse length. Rotor position is detected by comparing a value of current attained at the end of this fixed period of time to a preset value. The preset value is set by adjusting a potentiometer. When the preset value is exceeded, the phase is energized. This technique has a disadvantage in that the current at the end of the fixed period of time must be increasing for each successive clock pulse. The inductance of a phase winding that is about to be energized, however, can also decrease while the previous phase is still being driven. Thus, additional complexity is required in the drive circuit for determining whether the inductance is increasing or decreasing. Another disadvantage of this technique is that the preset value must be calibrated before the motor can be driven. Thus, variations in the observed current values, such as those caused by variations in applied motor voltage, drive current, load and variations in winding values from phase-to-phase, will result in each commutation occurring at various times relative to rotor position.
U.S. Pat. No. 4,868,478, discloses an energizing system for a switched-type reluctance motor. High frequency pulses of short duration are fed to power transistors in the energizing circuit of that phase winding which is next in line for excitation. Commutation to the next phase is performed by comparing observed values to a preset value, similarly to U.S. Pat. No. 4,520,302, described above. U.S. Pat. No. 4,868,478, however discloses that a correction factor is calculated in an attempt to account for variations in the applied motor voltage, drive current and load. Accordingly, this technique does not overcome all the limitations of U.S. Pat. No. 4,520,302. In addition, the expense and complexity of the system disclosed by U.S. Pat. No. 4,868,478, is increased by its suggested use of a microprocessor.
Despite the advantages of switched reluctance motors, the difficulties encountered in controlling these motors has resulted in their not being as widely utilized as they might otherwise be. Therefore, what is needed is an improved circuit for controlling a switched reluctance motor and that does not require use of a rotor position sensor.